Cooling pumps at one of Japan's crippled nuclear reactors are damaged beyond repair and will need to be replaced, officials learned Monday.

The revelation dashed hopes for a quick resolution to the ongoing nuclear catastrophe at the leaking Fukushima Dai-ichi plant.

An emergency order has been placed for new pumps for Unit 2 at the plant, but it's unclear how quickly they would arrive, officials said.

Engineers have worked around the clock to restore power to the facility, but damage caused by the earthquake and tsunami means it may take weeks to repair the required systems, officials warn.

"We have experienced a very huge disaster that has caused very large damage at a nuclear power generation plant on a scale that we had not expected," said Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency.

Although some headway was made toward bringing less-affected reactors on-line, conditions at the plant remained volatile.

Workers were temporarily evacuated Monday after plumes of mysterious gray smoke rose from a leaking reactor.

Officials said there was no sign of an explosion and that they had not detected any rise in radiation levels.

Meanwhile, officials with the U.S. Nuclear Regulatory Commission upgraded their assessment of the situation at the plant, saying it appeared the reactor cores at the most damaged facilities remained contained.

"I would say optimistically that things appear to be on the verge of stabilizing," said Bill Borchardt, the NRC's executive director for operations.

More than 18,400 are thought to have been killed in the devastating 9.0 quake and tsunami on March 11. Nearly half a million people were displaced.

With the nuclear crisis far from over, officials have detected food tainted by radioactivity over a wider area.

Trace amounts of radioactive iodine has been detected in spinach and other greens in the area around the plant, as well as in the water supply.